1. Field of the Invention
The present invention relates to load cells, used particularly in the construction of weighing scales such as bathroom scales, food scales or any other weighing apparatus with a platform.
Platform weighing appliances are generally formed of a base resting on the floor and a platform substantially parallel to this base. The platform is intended to receive the load to be weighed.
2. Description of the Prior Art
Between the base and the platform a load cell is disposed comprising strain gauges connected electrically in a Wheatstone bridge arrangement and delivering an electric signal proportional to the load applied to the platform. The load cell comprises a resiliently deformable flexion bar disposed in a direction substantially perpendicular to the force to be measured, i.e. substantially parallel to the platform. A mobile upright connects the platform to a first end of the flexion bar, the mobile upright being generally parallel to the direction of the force to be measured. A fixed upright, also generally parallel to the force to be measured connects the base to the second end of the flexion bar. The strain gauges are fixed on the surface of the flexion bar and are sensitive to the extension and shortening of said surface generated by the flexion of said bar. Such a structure is described for example in the document FR-A-2 177 923.
The difficulty with load cells resides in the fact that the electric signal which is obtained is sensitive to the position of the load on the platform. In fact, the flexional characteristics of the flexion bar vary both as a function of the longitudinal off-setting of the load on the platform, or position in the length direction of the flexion bar, and as a function of the transverse off-set of the load on the platform, or position in the direction perpendicular to the length of the flexion bar. The longitudinal displacement of the load introduces a variation in the torsion torque applied to the first end of the flexion bar by the mobile upright along a longitudinal axis. These torsional stresses deform the flexion bar and disturb the measurement of its flexion by the strain gauges.
In the document FR-A-2 177 923, the effects of the longitudinal displacement of the load on the platform are compensated for by disposing several strain gauges in at least two zones offset longitudinally on the flexion bar. The device however remains sensitive to the displacement of the load in a transverse direction.
Load cells are known made from a single block and machined so as to make them insensitive to the transverse off-setting of the load. Such load cells, described for example in the document EP-A-0 153 121 or EP-A-0 089 209 are however very costly because of the precise machining which they require. Such machining must be achieved by successive approximations and depends on a large number of parameters, making large scale manufacture without re-treatment illusory.
The document WO-A-8 402 186 teaches making such a load cell insensitive to the transverse offset of the load by using additional strain gauges sensitive to the torsion of the flexion bar. This approach involves using additional gauges, which must be disposed very accurately on the flexion bar and requires subsequent adjusting of the electric signals. Experience shows that such adjustment is very often insufficient and requires precise machining of the flexion bar. Furthermore, it is necessary to increase the number of strain gauges, which increases the connection to be made during fitting, increases the risks of breakdown and substantially increases the cost.
The document CH-A-658 909 teaches reducing the measurement differences due to offsetting of the load by using two blades (A1, A2) of the same lengths (1), parallel to each other, of the same width and spaced apart by a constant distance (A). Two straib gauges (D1, D2) are disposed on the first blade (A1) in spaced positions towards the ends of the blade. It is however apparent that such a two blade structure substantially reduces the deformation undergone by the assembly of the two blades (A1, A2) for the same flexional force and divides the sensitivity of the device. Furthermore, the presence of the two flat blades substantially increases the damping time of the oscillations when a load is placed on the device, which considerably affects its use.
Thus, the known solutions require expensive adjustment during the manufacture of the weighing appliances, or reduce the sensitivity and the mechanical damping capacities of the load cell.
A problem set by the present invention is to construct a strain gauge load cell, of simple design and structure so as to be of a low cost price, which has good sensitivity and a low damping time of the oscillations and which, by construction, is practically insensitive to the offset of the load, longitudinally and/or transversely.
According to the invention, the advantages are obtained without increasing the number of strain gauges and without requiring precise machining adjustments during manufacture. Furthermore, another problem set by the invention is to obtain insensitivity to the longitudinal offset of the load while avoiding dispersing the strain gauges in several zones spaced apart over the length of the flexion bar. Such dispersion in fact complicates the electric connection operations during mounting. Thus, the invention makes it possible to gather together all the strain gauges in a single zone of the flexion bar.
For that, a first idea which is at the base of the invention consists in using a load cell with two flexion beams, by specializing the two flexion beams so that each one fulfils particular functions : the first relatively thick and narrow beam, carrying the strain gauges, increases the sensitivity of the measurement and increases the mechanical damping; a second beam, relatively thin and wide, takes up the torsional forces along the longitudinal axis without exaggeratedly reducing the measurement sensitivity.
A second idea at the base of the invention results from the surprising observation that the effects of longitudinal offsetting of the load are appreciably modified when the two flexion beams are not parallel to each other.